This invention relates to an optical modulator, comprising a layer of an electro-optic material being provided between an integrated circuit and a light transmissive sheet. It also relates to an integrated circuit for use in such a modulator and a driving scheme for such a modulator.
Such optical modulators are known as silicon backplane modulators when the integrated circuit used comprises a silicon chip, but any semiconducting or semi-insulating material can form the body comprising the integrated circuit.
Liquid crystal silicon backplane devices are optical modulators of the above type in which the electro-optic layer comprises a liquid crystal layer provided directly on top of a silicon memory chip. A fuller description of these devices is contained in other documents such as EP-A-548180 and EP-11-548179. The integrated circuit is usually either an SRAM (static random access memory) type or a DRAM (dynamic random access memory) type, although other types of circuit are possible. There are many implementations of the silicon backplane device using an SRAM substrate, but all tend to have at least the basic structure shown schematically in FIGS. 1 and 2. FIG. 1 shows a block diagram of the silicon backplane. It consists of 2 shift registers one of which is connected to the xe2x80x98dataxe2x80x99 lines and one of which is connected to the xe2x80x98enablexe2x80x99 lines, via latches, of the SRAM array. Serial data is fed into the shift registers and xe2x80x98clockedxe2x80x99 along until the whole shift register is filled with valid data. The valid data is then written into the array of SRAM elements when the latches on the enable lines are enabled. It is usual that only one row of the SRAM array is enabled and the rest of the rows disabled so that data is written into the rows a line at a time. Data is then loaded into the shift registers for another row in the SRAM array.
FIG. 2 shows the block diagram of an SRAM pixel. The SRAM block output is either held high or low depending on the data that was last loaded into the SRAM. Data can only be loaded in when the enable line is held high. When the enable line is low any data presented at the input is ignored. Often there is an exclusive xe2x80x98ORxe2x80x99 gate (XOR) between the liquid crystal element and the SRAM element so that the output from the SRAM can be easily inverted by an xe2x80x98invertxe2x80x99 signal without needing to reload the inverted data into the SRAM. This is useful if the electro-optic material is a ferroelectric liquid crystal which requires charge balanced drive pulses. The invert signal is usually a xe2x80x98globalxe2x80x99 signal in that all the xe2x80x98invertxe2x80x99 signals are connected together, so that all the pixels are inverted simultaneously.
If a ferroelectric liquid crystal (FLC) is used, its bistable-memory effect degrades unless the applied electric fields are dc balanced on average. This is normally achieved by writing a frame of information and then inverting this image (using the global invert signal) and displaying it for the same period of time as the original, non-inverted image, so dc balancing every pixel over a period of 2 frames. If the device is illuminated during both these time periods, then obviously the image xe2x80x98washes outxe2x80x99. To avoid this, the illumination source is modulated so that the device is not illuminated during the time the inverted image is displayed. Clearly this reduces the amount of time for which the image can be usefully displayed and so the average brightness is relatively low.
As the silicon backplane usually only produces positive voltages O-Vd (Vd usually=5 V), the dc compensating negative voltage is generated by either holding the front electrode at Vd/2 (so that the liquid crystal experiences both positive and negative voltages) during both frames, or by holding the front electrode at 0 V during the writing of the non-inverted image and then holding it at 5 V during the writing of the inverse image.
The liquid crystal can either experience just one voltage polarity in one frame or both polarities in one frame but at half the voltage. Since the switching speed of most ferroelectric liquid crystals are very sensitive to the applied voltage (if the applied voltage is reduced from say 5 V to 2.5 V the switching speed can halve or worse.) it is preferable to operate the device with the higher voltages so that the devices can be operated at fast frame rates so that time-dither greyscale can be used. However, if an efficient time dither greyscale is used (such as that described in EP-261901) then this requires both positive and negative voltages to be applied within the same frame. This is not possible with the present design of silicon backplane devices. The invention aims to alleviate this problem.
According to a first aspect of the invention, there is provided an optical modulator comprising a layer of an electro-optic material being provided between an integrated circuit and a light transmissive sheet. The integrated circuit carries electrodes which cooperate with selected regions of said layer, the electrodes being addressed in use with data according to an addressing sequence, which sequence is repeated in successive time periods. The light transmissive sheet carries one or more light transmissive electrodes. The optical modulator includes means for providing both a positive and a negative voltage across said layer in a given time period. The optical modulator includes means for providing a plurality of voltage pulses to the one or more light transmissive sheet in a given time period.
The above optical modulator further includes drive means for providing a plurality of positive voltage pulses and a plurality of negative voltage pulses across said layer in a given time period. Also, the integrated circuit carrying the electrodes is provided with a first plurality of row conductors each coupled to a subset of said electrodes and a further plurality of column conductors each coupled to a different subset of said electrodes. Means are provided to provide different patterns of voltage pulses during a said time period to different members of both the first plurality or row conductors and the further plurality of column conductors.
Advantageously, the electro-optic material is stable in a of states having respective optical properties. Further advantageously, the electro-optic material comprises a ferroelectric liquid crystal.
According to a second aspect of the invention there is provided an integrated circuit for use in the above optical modulator. In the integrated circuit, the means to provide different patterns of voltage pulses comprises a pair of shift registers each having a plurality of outputs, respective shift registers being capable of being coupled in use to respective members of said first plurality of row conductors or said further plurality of column conductors. Alternatively, the means to provide different patterns of voltage pulses comprises a row decoder and a column decoder, the decoders being capable of being coupled in use to respective members of said first plurality of row conductors or said further plurality of column conductors.
According to a third aspect of the invention there is provided a driving scheme for an optical modulator in which a method addressing an electro-optic modulator having an integrated circuit forming a boundary on one side of the electro-optic layer and a light transmissive substrate carrying one or more light transmissive electrodes on the other side of the electro-optic layer, includes the step of applying a plurality of voltage pulses in a given frame time period to the electrode(s) being carried by the light transmissive substrate. Advantageously, each of the plurality of voltage pulses have the same polarity.